With the need for reduced power consumption, a thinner width and lighter weight for recent liquid crystal display devices, surface light source devices for illuminating the liquid crystal display device from the rear side are also required to be thinner and lighter. In order to meet these needs, power consumption by the light source is reduced by more effectively using the light from the light sources. An edge-light type surface light source device and a direct-light type light source device are known as the surface light source devices used in such liquid crystal displays.
In the edge-light type surface light source device, in general, light from the light source enters from a side end surface of a plate-shaped light guide body made of a transparent acrylic resin or the like, and exits from a light exiting surface, which is another surface of the light guide body, to the rear side of the liquid crystal panel and the like. In this surface light source device, the light usage efficiency is increased by providing a light reflection plate or a light reflection film on a surface of the light guide body that is opposite from the light exiting surface. In addition, the exiting light is made uniform by providing a light diffusion sheet on the side of the light exiting surface of the light guide body. On the other hand, in the direct-light type surface light source device, a liquid crystal panel and a reflection plate are arranged across the light source provided therebetween. The exiting light is normally made uniform by reflecting the light from the light source by the reflection plate to the backside of the liquid crystal panel and diffusing the light by a light diffusion sheet positioned on the light entrance side of the liquid crystal panel.
In such edge-light type or direct-light type surface light source device, an optical sheet, on which a plurality of unit prisms is arranged, is positioned on the light exiting surface of the light guide body. The optical sheet refracts and transmits the light from the surface light source on the side of the display body, such as a liquid crystal panel. However, such an optical sheet generates nonuniformity of brightness, such as uneven brightness by equal thickness fringes and visible images of the light source, or repeated markings of bright and dark areas, caused by the light from the surface light source device. The liquid crystal display device equipped with such an optical sheet encounters a problem that the images easily become distorted by the nonuniformity of the brightness.
To solve such a problem, Patent Reference 1 below proposes an optical sheet having fine grooves on a smooth surface opposite from the surface on which a plurality of unit prisms are provided. With this optical sheet, however, there is a problem that the primary function of the optical sheet, which is, increasing the brightness of the light by collecting the light from the surface light source device in a direction normal to the light exiting surface, is decreased. Patent References 2 and 3 below propose an optical sheet, a surface light source device and a transmissive type display device that controls the generation of the fringes without decreasing the brightness on the light exiting surface side. Such an optical sheet is arranged with a plurality of unit prisms on the surface of a translucent base and covers the rear side of the translucent substrate by a coating layer made of a translucent material. In Patent Reference 2, a large number of fine bumps are provided on the surface of the coating layer opposite from the translucent base. In addition, Patent Reference 3 provides fine protrusions by including spherical beads in the coating layer. In both cases, nonuniformity of brightness that is caused by generation of fringes is suppressed.    [Patent Reference 1] Japanese laid-open application number H07-151909    [Patent Reference 2] Japanese laid-open application number H10-300908    [Patent Reference 3] Japanese laid-open application number H11-133214